The 1918 influenza pandemic

A deadly influenza pandemic infected about a third of the world's people. It was called Spanish flu because neutral Spain reported on it openly while wartime censors elsewhere did not.

Estimates of deaths vary widely.

Context

The 1918 influenza was caused by an H1N1 flu virus and arrived in several waves from spring 1918 to 1919, with the autumn wave the deadliest. About a third of the world's population is thought to have been infected, and deaths are estimated at 50 million or more. Unusually, many victims were healthy young adults.

It was not Spanish in origin. Newspapers in neutral Spain reported openly about the illness while governments at war censored news, so Spain seemed hardest hit, and the name stuck. Where the virus began is still debated.

There were no vaccines or antibiotics to treat the secondary infections. Cities tried closing schools and public gatherings, wearing masks and isolating the sick, and records from that period are still studied for what worked.

The pandemic is a reference point for modern public health. Today's pandemic planning, vaccines and global flu surveillance all trace back in part to lessons learned in 1918 and 1919. Later studies found that cities which acted early and kept restrictions in place longer generally recorded lower death rates.